This invention relates to the continuous preheating of a principally metallic charge to form a molten steel product. More particularly, it relates to continuous preheating of organic compound contaminated charge materials for continuous steelmaking in an associated electric arc furnace (EAF) which reduces noxious emissions generated by the furnace. Some regions of the world have very strict emission standards. The present invention address the problem steelmaking facilities have in complying with these standards.
Continuous steelmaking is particularly advantageous in those regions where there is a concentration of production of, or ready availability of scrap and/or direct reduced iron (DRI), and where electric energy is both available and economical.
Non-metallics from metal-containing charge materials often cause environmental problems, particularly when burned at low temperatures, which results in incomplete combustion. Under such conditions, the electric arc steelmaking furnace produces noxious emissions, including dioxins. These dioxins form in the furnace or in dust collection ducts from combustion of plastics from pipes and cars, synthetic fibers, paints, cutting oils from turnings, and the like. Dioxins form at temperatures between about 300.degree. and 520.degree. C., with the most favorable temperature for forming such compounds being about 420.degree. to 470.degree. C. Dioxins generally form in the presence of chlorine. The reaction is catalyzed by the presence of copper or other catalyst. In general, the temperature required for dioxin creation is from about 320.degree. to 520.degree. C. Dioxins form readily when combustion takes place at low temperatures. The use of chlorinated compounds in iron and steel processes and recycled scrap material contaminated with cutting oils and plastics containing chlorine, such as PVC, provides all the factors required for the formation of the chlorinated aromatic compounds polychlorinated dibenzo dioxins (PCDD) and polychlorinated dibenzo furans (PCDF).
Such harmful compounds are not produced during continuous scrap preheating and feeding in the present process, because the organic matter in the scrap charge is combusted at high temperatures before it reaches the furnace, and the atmosphere within the preheater is carefully controlled. The incineration chamber following the preheater provides sufficiently high temperature, sufficient turbulence, and an excess of oxygen which results in dioxins and precursors being destroyed. If the destruction of dioxins at this stage is complete, formation later on in the system is not possible.
Historically, the operation of an electric arc steelmaking furnace has been an intermittent operation, wherein the sequence followed is: charging of steel scrap and/or direct reduced iron, pig iron, slag formers and alloying elements; ignition or establishment of an electric arc between the electrodes in the furnace to create melting conditions for melting the charge and forming a molten metal bath covered by a molten slag; refining for a period of time during which the molten metal portion of the bath is refined to form steel having a desired composition and quality; and periodically raising the electrodes to remove them from contact with the bath and interference with the tapping procedure; and then tapping the molten metal. In addition, slag can be removed by a deslagging, or slag-off, operation as required.
The present invention is particularly well suited for use in the continuous steelmaking process described in my U.S. Pat. Nos. 4,543,124 and 4,609,400, which are incorporated herein by reference.